Directional microphone



J. C. BLEAZEY DIRECTIONAL MICROPHONE,

Nov. 26, 1957 2 Sheets-Sheet 1 Filed Oct. 28, 1954 INVENTOR. Jul-IN E. BLEAZ EY 1 ATTORNEY Nov. 26, 1957 J. c. BLEAZEY 2,814,677

DIRECTIONAL MICROPHONE Filed Oct. 28, 1954 2 S heets-Sheet-2 mmumcy java INVENTOR. Jun-m E. BLEAZEY BY Z "IT DRNEY United States DIRECTIONAL MICROPHONE John C. Bleazey, Mercerville, N. 1., assiguor to Radio Corporation of America, a corporation of Delaware This invention relates to directional microphones, and more particularly to means for improving and adjusting the response of such microphones.

A unidirectional ribbon type microphone is designed to attain a certain directional response. In designing such a microphone, the acoustic signals emanating from a vertical plane with respect to the microphone axis are generally given the primary consideration. As a result, the response of such a microphone for acoustic signals originating from the horizontal plane is often not given due consideration. In such a case, the unidirectional microphone may have a cardioid response characteristic for sounds received from a vertical plane, and a somewhat difierent response characteristic for sounds received from a horizontal plane. In the foregoing, the microphone is disposed so that its axis, which corresponds to a line normal to the vibratile member, is vertical.

While uniform directional characteristics for sounds originating from different planes are desirable in first order gradient microphones, it is fundamental to the operation of second order gradient microphones. This is true since second order gradient microphones, and other higher order gradient microphones, obtain their high directivities by matching the responses of two polydirectional microphones and connecting their electrical outputs in opposition. The directional characteristics of microphones used in higher order gradient operations should match as closely as possible to attain satisfactory operation.

In higher order gradient unidirectional microphones, it is dilficult to design more than one microphone having exactly the same directional characteristics. It is especially difiicult to design a number of microphones having the same response characteristics for sounds emanating from different planes with respect to the microphones. For this reason, it is desirable to have an easy means for adjusting the characteristic response of a microphone after its main mechanical elements have been assembled.

Very often, a pair or more of microphones may have the same response for sound received at zero degrees with respect to their axes. However, for other angles, such as ninety degrees, the response of the microphones may dilfer. In such cases, it is desirable to have means for adjusting the responses of the microphones for ninety degrees, for example, without aifecting the responses at zero or one hundred and eighty degrees.

It is an object of this invention to provide an improved 7 microphone having substantially the same response characteristics for sounds originating from diiferent planes with respect to the microphone axis.

It is a further object of this invention to provide an improved microphone especially adapted for second order gradient operation.

It is still a further object of this invention to provide an improved microphone with means for adjusting the response of said microphone for sounds emanating from different planes with respect to the microphone axis.

It is still a further object of this invention to provide an improved means for adjusting the response of a microphone for a predetermined angle without affecting the response for other angles.

In accordance with the present invention, a ribbon type microphone having an acoustic terminating device is provided. The microphone includes a magnetic structure having a plurality of magnets and a pair of pole pieces spaced to provide an air gap therebetween, within which the ribbon of the microphone is adapted to vibrate. An acoustic pipe provides an air chamber at the rear surface of the ribbon and is connected to the acoustic terminating device. The pipe is provided with a series of apertures. The magnets and the pole are cut away to provide substantially direct acoustic coupling between the ambient and the apertures within the acoustic pipe for sounds originating from both a horizontal plane and a vertical with respect to the microphone axis.

Other objects and advantages of the present invention will be apparent from a reading of the following specification in connection with the accompanying drawing, in which:

Figure 1 is a front view, partly in cross-section, of a ribbon type microphone embodying the present invention;

Figure 2 is a cross-sectional view, partly broken away and with the top cover removed, of the microphone shown in Figure 1, taken along the line 2-2, as viewed in the direction of the arrows;

Figure 3 is a top view of the microphone shown in Figure 1, with the top cover removed;

Figure 4 is a side View similar to Figure 2 but being modified by inclusion of the means for adjusting its response;

Figure 5 is a top view of the microphone shown in Figure 4; and

Figure 6 is a set of characteristic curves illustrating the responses of a microphone utilizing the adjusting means as shown in Figures 4 and 5.

Referring particularly to Figures 1, 2 and 3 of the drawing, a ribbon type microphone 10 has a cover 11 and includes a magnetic structure 12. The magnetic structure comprises a plurality of magnets 14, 16, 18 and 20 having one of their ends suitably attached to a bottom plate 22. The magnets are located around the central axis of the microphone .and are circumferentially equally spaced. Each of the magnets are tapered, being narrow toward the top and wider toward the bottom. A pair of pole pieces 24 and 26 are disposed at the other ends or tops of the magnets. The pole pieces are spaced to provide an air gap therebetween. The pole piece 24 is attached to the magnets 16 and 18 and the pole piece 26 is attached to the magnets 14 and 20. Each of the pole pieces have cut-away portions, the portions being substantially of the same width as the spacing between the magnets at one end and being tapered as it extends toward the air gap, substantially as shown.

A vibratile element or diaphragm 30, which may be a corrugated aluminum ribbon, is suitably attached between the pole pieces and disposed to vibrate within the air gap.

An acoustic terminating device in the form of an acoustic labyrinth 32 is provided. An acoustic pipe or conduit 34 is adapted to be secured to the labyrinth so that a continuous sound pathway is provided leading from the area of the back surface of the ribbon 30 to one end of the labyrinth. The labyrinth provides an acoustic resistance toward the rear surface of the ribhem and may be in the form of a long pipe having ozite or other sound absorbing material packed therein. The acoustic pipe 34 is provided with a series 9.1? side aper,

tures 36 and end apertures 35. The sizes of the apertures 35 and 36 are dimensioned to provide the desired response characteristic. The apertures may be thin horizontal thin openings, as shown, circular openings or other desirable shapes. In many unidirectional microphones, the apertures are proportioned to provide a cardioid response.

In the arrangement described, the magnets are spaced so that the ambient is directly coupled to the apertures 36 from substantially all angles to the microphone axis. If desired, a pair of unitary magnets each having cut-away portions to provide spaces corresponding to the space between the magnets 16 and 18 and the magnets 14 and 20 could be employed to attain this direct coupling, instead of the four magnets shown. Also, in some cases, it may be desirable to use more than four magnets. The pole pieces are cut away so that sounds originating from a plane horizontal with respect to the axis of the microphone and from the top of the microphone are not obstructed. Since the magnetic structure ofiers little ohstruction to the sound waves, the response of the microphone is more uniform for sounds originating from both horizontal and vertical planes with respect to the microphone axis. The series or plurality of apertures in excess of two in the acoustic pipe provides the means for a substantially direct coupling path between the sounds originating from the ambient and the rear surface of the ribbon. The apertures are disposed substantially at all angles to the microphone axis.

Thus it is seen, that the arrangement comprising the cut away pole pieces and magnets in the magnetic structure, together with the acoustic pipe having a series of apertures, provide substantially direct coupling for sound waves arriving at the rear surface of the ribbon. This direct coupling is substantially the same for sounds arriving from horizontal and vertical planes with respect to the microphone axis. The arrangement facilitates the design of microphones which are to have the same response characteristics for sounds arriving from different planes. For example, by following teachings of the present invention, a microphone may be designed to have a cardioid response characteristic for sounds arriving from different planes.

The electrical output from the microphone is connected by means of a cable 39 to a utilization circuit (not shown). The conductors which lead from the ribbon to a transformer and from the transformer to the cable 39 in the .usual manner are not shown.

In designing second order gradient microphones, two polydirectional microphones may be employed, with the electrical outputs of such microphones connected in opposition. It is necessary that the acoustic characteristics of the polydirectional microphones be substantially equal to provide uniformity of frequency response. Very often such microphones will have substantially the same response for sounds emanating from zero degrees with respect to the microphone axis. However, for sounds emanating from ninety degrees with respect to the micro phone axis, for example, the directional characteristics often vary.

By varying the acoustic resistance with respect to the acoustic capacitance in a circuit of a'polydirectional microphone, the response of the microphone at ninety degrees, for example, maybe shifted about four decibels without changing the zero degree response, or materially changing the one hundred and eighty degree response. The present invention provides means for varying the response of the microphone for a certain angle without materially affecting the zero or one hundred and eighty degree responses.

Referring particularly to Figures 4 and 5, there is illustrated means for varying the acoustic resistance with respect to the acoustic capacitance. The magnetic struc ture shown in Figures 4 and is substantially the same as that illustrated in Figures 2 and 3 and, therefore, the

same reference characters are used for corresponding parts. In this embodiment cotton or ozite 38, or other sound-absorbing material, is stuffed in the spaces between the four magnets, as shown. This material surrounds the acoustic pipe and is disposed in substantially all the space between the top and bottom plates of the magnetic structure.

Referring particularly to Figure 6, the eifect of the sound-absorbing material, which may be considered as an acoustic resistance, on the directivity pattern of the microphone, such as the one illustrated in Figure l, is shown in the form of a series of characteristic curves. The curve 4-6 illustrates the response of the microphone for acoustic signals arriving at zero degrees with respect to the microphone axis. The curve 42 illustrates the response of the microphone for sound emanating from ninety degrees from the microphone axis. This curve illustrates the response when the sound absorbing ma" terial is stuffed tightly between the magnets. The curve 44 illustrates the response of a microphone for sound emanating from ninety degrees to its axis with the cotton stuffed lightly between the magnets. A curve 46 illustrates the response of a microphone to sounds emanating one hundred and eighty degrees from the microphone axis. It is seen from these curves that if the zero degree response is at a 30 db level that the normal ninety degree response would be at 24 db varying the density of the acoustic absorbing material will change the ninety degree response plus or minus 2 db, or between 22 and 26 db.

It is seen that if two or more microphones are combined to provide a higher order gradient microphone, the differences in response characteristics at ninety degrees may be readily adjusted by stuifing acoustic absorbing material lightly or tightly in the space between the magnets. This arrangement provides a simple means for matching the response characteristics of two polydirectional microphones, and is especially adaptable for higher gradient microphones.

The particular magnetic structure, together with response adjusting means, embodying the present invention, provides a microphone which is ideally suited for the construction of higher order gradient sound systems. The invention is also applicable to a unidirectional micro phone having a cordioid response characteristic. In this case, a cardioid characteristic for horizontal and vertical planes is possible due to the novel arrangement of the magnetic structure together with the novel arrangement of the apertures in the acoustic pipe.

What is claimed is:

1. In a microphone having an acoustic terminating device, the combination comprising a magnetic structure providing an air gap, a vibratile element having front and rear surfaces disposed to move within said air gap, means providing an air chamber having ends and sides with said rear surface of said vibratile element, said means coupling said rear surface of said vibratile element to said acoustic terminating device, and said means being provided with a plurality of apertures in excess of two which are disposed in said ends and in said sides of said chamber provided by said means.

2. In a microphone having an acoustic labyrinth, the combination comprising a magnetic structure providing an air gap, a vibratile element having front and rear surfaces disposed to move within said air gap, 21 pipe providing an air chamber at the rear surface of said vibratile element, said pipe leading from said rear surface of said vibratile element to said acoustic labyrinth, said pipe being provided with a series of apertures in excess of two, and said apertures being disposed substantially 360 about the axis of said microphone.

3. In a microphone having an acoustic labyrinth, the combination comprising a magnetic structure, said magnetic structure including a bottom plate, a pair of magnets and a pair of pole pieces, said pole pieces providing an air gap therebetween, a vibratile element having front and rear surfaces disposed to move within said air gap, a conduit providing an air chamber at the rear surface of said vibratile element, said conduit leading from said rear surface of said vibratile element to said acoustic labyrinth, said conduit being provided with a plurality of apertures, and said magnets being cut away to provide substantially direct acoustic coupling between said apertures and the ambient for substantially 360 about the axis of said microphone.

4. The invention as defined in claim 3 wherein said pole pieces are cut away to provide substantially direct acoustic coupling between the ambient at a plane substantially vertical to said axis of said microphone.

5. In a microphone having an acoustic labyrinth, the combination comprising a magnetic structure, said magnetic structure including a bottom plate, a pair of magnets and a pair of pole pieces, said pole pieces providing an air gap therebetween, a vibratile element having front and rear surfaces disposed to move within said air gap, a conduit providing an air chamber at the rear surface of said vibratile element, said conduit leading from said rear surface of said vibratile element to said acoustic labyrinth, said conduit being provided with a plurality of apertures, said magnets being cut away to provide substantially direct acoustic coupling between said apertures and the ambient for substantially 360 about the axis of said microphone, and response adjusting means disposed between said magnets.

6. In a microphone having anacoustic labyrinth, the combination comprising a magnetic structure, said magnetic structure including a bottom plate, a pair of magnets and a pair of pole pieces, said pole pieces providing an air gap therebetween, a vibratile element having front and rear surfaces disposed to move within said air gap, a. conduit providing an air chamber at the rear surface of said vibratile element, said conduit leading from said rear surface of said vibratile element to said acoustic labyrinth and being substantially disposed between said magnets, said conduit further being provided with a plurality of apertures, said magnets being cut away to provide substantially direct acoustic coupling between said apertures and the ambient for substantially 360 about the axis of said microphone, and response adjusting means disposed between said magnets and said conduit, said adjusting means including an acoustic absorbing material.

7. In a microphone having an acoustic labyrinth, the combination comprising a magnetic structure, said magnetic structure including a bottom plate, a pair of magnets and a pair of pole pieces, providing an air gap therebetween, a vibratile element having front and rear surfaces disposed to move within said air gap, a conduit providing an air chamber at the rear surface of said vibratile element, said conduit leading from said rear surface of said vibratile element to said acoustic labyrinth and being substantially disposed between said magnets, said conduit further being provided with a plurality of apertures in excess of two, said magnets being cut away to provide substantially direct acoustic coupling between said apertures and the ambient for substantially 360 about the axis of said microphone, said pole pieces being cut away to provide substantially direct coupling between the ambient at a plane substantially vertical to said axis of said microphone, and response adjusting means packed between said magnets and said conduit, said adjusting means including material comprising an acoustic resistance.

References Cited in the file of this patent UNITED STATES PATENTS 2,301,638 Olson Nov. 10, 1942 2,512,467 Olson et a1 June 20, 1950 2,680,787 Olson et a1. June 8, 1954 

